Process for pelletizing glassmaking materials



n Nov. 24, 1970 CZKLE'T JUNNOSUKE YAMAMOTO PROCESS FOR PELLETIZINGGLASSMAKING MATERIALS Filed March 10, 1969 I N V E N TO R tfan/7051K?)6pm/770i@ U.S. Cl. 65--27 4 Claims ABSTRACT F THE DISCLOSURE Processfor increasing the speed and uniformity in the production of glasswherein the glass constituents, mainly silica, limestone, sodiumcarbonate and sodium hydroxide, with sodium hydroxide furnishing atleast'20% by weight of the Na20 equivalent present, are initiallypulverized to fine powders having particle sizes within preferredranges, far smaller than customary in the art, preferably less than 0.42mm., with 60% to 65% thereof finer than about 0.15 mm. Then the powdersare mixed and pelletized, without the use of special binders or of hightemperatures or pressures, by tumbling or rolling in the presence of 12%to 20% by Weight of water, and the pellets are dried, preferably byheated air, initially at a temperature of 50 C. to 200 C. and withsubsequent drying temperatures not exceeding 700 C. The dried pellets,having a size of 4 to 20 millimeters in the largest dimension, and mixedwith cullet if desired, are fed to a furnace and rapidly converted toglass of high quality.

This application is a continuation-impart of my copending applicationSer. No. 486,546 led Sept. 10, 1965 and now abandoned.

This invention relates to a process for glassmaking.

In the usual glassmaking processes, the addition of the raw materials inthe form of rather coarse powders involves a great many disadvantages.Thus it is quite common that part of the material powders, which areprepared and carried to a melting furnace, disperses in a cloud of rdust and soils the surroundings and the working place for glassmaking.Also, in the conveyor, or particu-r larly in the bunker above the intakeof the furnace, the powders become segregated in composition, resultingin vitriication delays in the melting furnace and, in far too manycases, loss of homogeneity of the glass product. Furthermore, dispersionof powders thrown into the furnace destroys the upper construction ofthe combustion chamber and the refractory blocks in the heataccumulation chamber and the heat conversion chamber. Sometimes, thepowders clog the ilues of these chambers and cause serious damage to themelting furnace which requires entire replacement of packingrefractories. Also, variation in the water content of the powders oftenaffects the temperature in the furnace and in no small degree damagesthe product by the formation of bubbles and stones.

United States Patent O Heretofore, a method has been proposed forpreventing the dispersion of the powder dust by providing constanthumidity for the powders with water. This avoids the dispersion ofpowders in the furnace. However, this method has produced nosatisfactory results as yet.

Other attempts have been made to use alkali-resistant electrocastrefractories at the side wall of the upper furnace structure, but thereis no known way to prevent corrosion of the crown silica. Also, whilemagnesia refractories for the chamber are quite resistant to alkalidust, they cannot prevent undesirable condensation of alkali sulphate.

Speed of vitrication, that is, glass forming speed, has advanced withimprovements in furnace construction, efciency of operation byautomation, and improved quality of refractory blocks, but thedevelopment seems to have reached a limit under present technicalconditions. It was also thought that treatment of residual silica grainis most important since it delays reaction. Another proposal was topulverize grains much smaller in size than ordinary, but it isimpractical to pulverize` them further and put them into a continuousmelting furnace because of the above-mentioned disadvantages.

The pelletizing method for the granulation of powders has previouslybeen employed in the manufacture of iron and steel by use of pressingmolders, injection molding machines, drum-type granulators, and.pan-pelletizers for accomplishing the granulation under variousconditions. In general, however, such process has required a variety ofbinding agents to furnish pellets of sufficient strength. In thegranulation for glassmaking, however, the application of extraneousbinding agents is generally not suitable as it leaves heterogeneouscomponents in glass as residues of the binding agents which is liable tochange the composition of the glass and results in high cost ofmanufacture.

The present invention has for its object to provide a process forglassmaking which makes it possible to manufacture the glass bypelletizing the finely pulverized raw materials, whereby the glassproduct is produced at increased speed without changing the composition,but increasing the quality, of the product. This novel process is notexpensive and overcomes the disadvantages of previously knownglassmaking processes.

The present invention overcomes the disadvantages of the conventionalmehods of glassmaking by providing for pulverizing powders havingordinary grain size into much smaller particles and then pelletizingthem adequately to facilitate vitrifcation in the later stages ofglassmaking. As such pelletizing of the nely pulverized raw materialsfacilitates their melting, with the particles of silica, alkali,limestone and other ingredients in close contact, vitrfcaton occurs morerapidly and without change in the composition of the material. Thepellets of agglomerated powders for glassmaking will have the desiredstrength without the use of extraneous binding agents such as lignin orwater glass, which are unduly expensive or may change the composition.

With these and other objects in view, which will become apparent in thefollowing detailed description, the present invention will be clearlyunderstood in connection with the accompanying drawings, in which:

FIG. 1 is a ow sheet showing the essential process steps in accordancewith the invention; and

FIG. 2 is a diagrammatic View of a commercial installation for carryingout a perferred embodiment in accordance with the invention.

In accordance with this invention, the raw materials for glassmaking areinitially pulverzed to powders within a 4 The said pellets are readilymelted and rapidly dispersed in the glass melting furnace to produceglass of high quality.

Advantages attained in accordance with this invention are illustrated bythe results obtained in the following critical range of subdivision; thesubdivided powders are series of comparative experiments. In thisseries, the comthen thoroughly mixed, preferably by a dry mixingprocess; position of the ingredients and the melting condltions were thesubstantially uniform mixture of particles is then agidentical.glomerated to pellets having grain sizes within an opti- The Compositionof Sollds Was the following yPlCal mum range by subjecting the particlesto a tumbling acglilSSmaklIlg IIllXUrCZ tion, causing them to roll overone another repeatedly in Parts by wt. the presence of a critical rangeof water content; and the S111C a Sand 100 pellets are dried at atemperature within an Optimum Sodlum Carbonate 30 range, therebyproviding the raw material charge for the Limestone 30 glassmakingfurnace in a form which facilitates the vitril5 Glaubens Salt l cationProcess and furnishes the SnfPflSlng advantages 0f In Experiment 1 thepowders of ordinary coarse size thlS mVlllOIl. t were mixed and testedin that form, particularly as they The Critical ranges, Wlnell al eessenllal in obtaining the could not be well pelletized. In Experiments2 to 6, sodium advantages resulting from lJS IIIVeDOIl, are preSeIledcarbonate and limestone Were pulverzed into much lll the fOllOWlllgSummary and substantiated 111th@ detailed 20 Smaller grains thanOrdinary Size and mixed with silica description below. sand of variousgrain sizes. In each case, the mixture The faW materials, eXeeP slleaSand, must all be snbwas pelletized in the presence of 12-20% water anddried, divided initially to particles whose largest dimension is asdescribed above, liner than A.S.T.M. No. (0.420 mm.) with at least InExperiment 1, 200 grams of the ingredient powder 65% thereof being finerthan A.S.T.M. No. 100 (0.149 25 mixture, and in Experiments 2 5, 20()grams of the inmm) The silica Sand ingredient is all subdivided topargredient pellets, were placed in a Crucible, preheated at tiCleS1111er than A-STM N0- 50 (0.250 InIn-) With at 600 C. for thirtyminutes, and then held in an electric leaSt 60% thereO being lnef thanAST-M N0- 100 furnace at 1400 C. for varied periods for determining the(0149 Inn1) Subdivision t0 any SgDiCaIl further eXent minimum meltingtime required for complete melting and is not essential and may notprovide suiiciently increased 30 the disappearance of silica residues.The test piece was advantages to justify the added expense. annealed,cut to a thickness of about 7 to 8 mm., and was After the flueingredient particles have been thOrOugllly then ground for observation,mixed, they are readily agglomerated by rolling or tum- The followingTable I summarizes this series of combling aCtiOn in the presence of 12to 20% by weight 0f parative melting experilnents and the resultsthereof.

TABLE I Grain size 1,400c C.-Melting time, minutes Experiment .Allingredients Silica sand 15 25 35 45 75 105 135 165 Remarks (l)Bwslls2,31gmmlfj.About 15% Bgwliotntaffgl nml About 9% X X X X X X X 0Cdriiry powdery (2) Below' 2@mesh (84111,I11-);Ab011l3 34% be- Below 20mesh; About 9% below 10() X X X X X X A O Pefenal- 10W 100 mesh. mesh.(3) Below 40 mesh (.42 nlm); About 47% Below 40 mesh (.42 mm); About 30%X X X X A A O 0 D0 below 100 mesh. below 100 mesh. (4) Below 40 mesh;About 66% belOW 100 BelOW 60 HleSh (-25 IDBL); About 60% A O O O O Do.

mesh. below 100 mesh. (5) Below 40 mesh; About 80% belOW 100 BBIOW 80IneSh (.77 111111.); About 80% 0 O O O O Do.

mesh. below 100 mesh. (6) Berllfysvh 4o mesh; About 90% below 100 Below100 mesh (.149 mm.) O O O O O Do.

X Indicates significant residual silica. A Indicates slight residualsilica.

O Indicates no residual silica.

/ Experiment unnecessary.

water, about 15% on the average (15 parts by weight of water per 100parts by weight of particles). Within this critical range of moisturecontent, the agglomeration of particles to form rounded, compact andwell-bonded pellets of the desired size range and handlingcharacteristics proceeds smoothly and rapidly, a result which ispractically unattainable at either lower or higher water contents.

The drying of the pellets of agglomerated particles for the removal ofmoisture by evaporation must be eifected at initially moderatetemperature, above C. and below 200 C., preferably by means of heatedair or other gas maintained at a preferred temperature of 130 C. to 180C. The temperature in the dryer at the exit end must not exceed 700 C.,for substantially higher temperatures thereat would result in stickinessof the pellets and resulting difliculty or interruption in theirtransportation to the melting furnace.

When the process is carried out within the above-described criticalconditions, the resulting rounded pellets of agglomerated particles arebetween 4 and 20 millimeters in the largest dimension, and arecharacterized by excellent crush resistance and handlingcharacteristics.

Tables II to VII present the detailed screen analysis of the initialmain ingredients used in the experiments, in which the Glaubers salt(sodium sulfate) ingredient was the same standard commercial materialthroughout. Table II lists the grain sizes of the mixture ingredients asused in Experiment l, while Tables III to VII list the initial particlesizes of the ingredients, which were mixed and pelletized to roundedpellets of 4 to 20 mm. largest dimension for use in the meltingExperiments 2 to 6.

TABLE II.-EXPE RIMEN T 1 (conventional powder batch) TABLEIIL-EXPERIMENT 2 and including at least about 65% by weight of particlesfiner than ASTM No. 100, and silica sand ner than Material Silica sand'carll hummml Mea g ASTM .No. 60 and including at least about 60% byweight ASTM No percent percent percent percent Of paIIClCS filler thanASTM N01. 100, Which are granu- '0 5 lated into pellets in accordancewith this invention, en- 0.1 ables the attainment of far shorterbatch-free time comg pared with those ordinary grain size. Thus,complete 9.8 melting was rapidly obtained in Experiments 4, and 6 10'4in accordance with the invention. 66.5 10 In a second series of meltingexperiments, the ingredi- 8 3 ent powders having the same grain sizewere pelletized 25.5 as in Experiment 4. 200 grams of pellets were thenpre- 33'8 heated for 30 minutes at 600 C., and thereafter were treatedin an electric furnace at the temperatures and TABLE IV EXPERIMENT 3.llted in Table VIII. The results were as shown in Sodium Mean grainMaterial, Silica sand, carbonate, Limestone, size,wt. ASTM N 0 wt.percent wt. percent wt. percent percent TABLE VIII 0.2 0.2 Dug Meltingtime, min 15 25 35 45 75 39. 5 3. 8 2. 4 25. 9 Temperature, C.: 17.6 8.66.0 13.7 1,250 X X X A 0 12.7 15.2 13.2 13.3 X A o o A O O 0 Total 53.5O 0 O O 10G-120 11.8 17.2 10.4 12.6 25 X Significant residueof silica.120 17. 8 54. 8 67. 7 34. l A Slight residue of silica.

O N o silica residue. Total 46.7

It `was found that scums disappeared more quickly even TABLEV--EXPERIMENT 4 30 at low temperatures than with powders of ordinarygrain C l Sodium Mean grain size. This would lower the fuel cost andprolong the liiaiio @riferiti-225e vtlf $5353.85 cllt hf@ Offb@ meltmgfurnace 0 0 It will thus be clear that the present invention is based0:2 35 on the use of pellets of agglomerated particles whose meltingspeed 1s exceedingly rapid as compared with 16:8 powders of ordinarygrain size. Accordingly, this inven- 343 tion accomplishessignicantreductions in the cost of glassmaking. In this invention, the powderdust is not alglowed to ily about in the air and powders are not allowedto segregate during transportation, neither will dispersion 9 of dustsoccur in the melting furnace. In consequence, the invention permits themanufacture of homogeneous glass l products under improved workingconditions. It also per- TABLE VI.- EXPERIMENT 5 45 mits the use ofpotassium carbonate or dolomite instead Sodiufm Mean grain of sodiumcarbonate and limestone .for the same purpose. Material, Silica sand,carbonate, Limestone, size, wt. Refining agents may be used 1n additionto the main con- ASTM No ft-percent Vt-pement Wtpercent pementstituents, and a mixture of boric compound, phosphoric 0.0 compound,barium compound, and lead compound may (f 50 also be used in accordancewith the glass composition 3.1 required. 162 In a preferred embodimentof this invention, the raw 20.7 material ingredients were `initiallysubdivided so as to be 1&3 finer than ASTM No. 60 in mean grain size andto in- 6L2 55 clude over 70% by weight of a size liner than ASTM 79.5No. 100. The grain size of the :silica sand =was almost entirely belowASTM No. 60 with about 70% liner than ASTM No. 100, as shown in detailin Table IX. The powders were mixed with small amounts of additives andTABLE VH-'"EXPERIMENT 6 60 colorants. After thorough dry-mixing, roundedpellets of d sbodium Li t Mean graitn 4 to 2O mm. size were prepared bytumbling the mixture Material' Silica 5.a f car mate mes one' SZefw ofparticles in the presence of 12% to 20% by weight ASTM Naf WL pement Wt'percent wt'pemnt meent of Water, and drying at an initial temperature of130 C.

g-g io 180 C. i2 65 It has been found that in order to obtain pellets ofadequate strength, recourse was had to caustic soda, preferably inliquid form. Thus, it has been found that caustic 99 soda increased thebinding characteristics of the powder 10.8 ingredients to the extentthat the subsequently formed 79'6 70 pellets had the requi'sitestrength. 'The crushing strength 90.4 of the pellets was quite highafter the pellets were dried.

The above experiments show that the use of ingredient powders, exceptingsilica sand, finer than ASTM No. 40

The caustic soda may replace all or part of the sodium carbonate,preferably in a Na2O basis in excess of at least 20%. Caustic soda may,of course, be applied either as powder or in solution.

7 TABLE IX Parts by wt. Silica sand 100 Sodium carbonate 24 Caustic soda7.5

Limestone 30 Glaubers salt 1.0 Potassium bichromate 0.03 Cobalt oxide0.005 Carbon 0.08

Generally, in accordance `with the flow sheet illustrated in FIG. 1 ofthe drawing, the above components were mixed thoroughly and thengranulated in a pan pelletizer. The grains obtained were dehydrated atan exemplary initial drying temperature of 130 C. to 180 C., mixed with40 parts of glass cullets, and charged into a continuous glass meltingfurnace. After melting, there was obtained excellent blue colored glass.Also, 50% more glass was obtained than the maximum amount obtained whenordinary coarse powders were charged into the same continuous glassmelting furnace. It was further noted that the consumption of heatingfuel was significantly reduced by this pelletizing method.

TABLE X A more detailed showing of a commercial installation forcarrying out a preferred process in accordance with the invention isdiagrammatically presented in FIG. 2.

The finely pulverized ingredients are charged into storrage tanks 1, 2,3 and 4 provided, respectively, for silica Sand, soda ash, limestone,and the mixture of minor ingredients, such as Glaubers salt, cobaltoxide, potassium bichromate, and carbon.

The raw material ingredients are conveyed in the desired Weight ratios,for example by the use of constant feed weighers or a conventionalintermittent weighing system, and discharged into mixer S of any knowntype which efliciently and thoroughly agitates the dry particles toproduce a substantially uniform mixture thereof.

The mixture is delivered for further mixing into pug mill 6, at whichcaustic soda in liquid state of preferably over 49% concentration,together with a certain amount of water, is added. As the moist premixof particles passes through the mill, the complete mixing of the liquidand particles takes place on their passage through the mill.

The moist mixture is delivered to the granulating apparatus, preferablya disk pelletizer 7 of known type, comprising a rotating inclined panprovided with a iiow adjusting rod, suitable baffles, bottom and sidescrapers and other means for smooth grain production. Sufficientadditional water may again be added to the mixture to insure themaintenance of the total moisture content of the mixture within therange of 12% to 20% by weight.

The agglomerated pellets overflow from the edge of the rotating disk andpass through a re-roller 8, which may consist of an inclined rotarydrum. This completes the formation of compact rounded pellets of theagglomerated particles of the raw materials in the desired size range,which are discharged into a suitable dryer 9. Drying temperatures lowerthan the aforementioned initial 130 C. to 180 C. require prolongedperiods for the evaporation of the Water content. On the other hand,initial drying temperatures higher than 200 C. produce adverse effectsthrough sudden thermal shock tending to cause cracks which significantlyreduce the crushing strength of the pellets and impair the handlingcharacteristics.

The dried pellets are then screened in sieve 10 which delivers producthaving the desired optimum size within the range of 4 to 20 millimetersmaximum dimension. Oversize pellets are subdivided in pulverizer 11 andreturned together with undersize pellets, to the mixer 5, or even to thepug mill 6 of pelletizer 7. The 4 to 20 mm. pellets are charged into theglass furnace 12, together with admixture of glass cullets if desired,or may be delivered to storage for future use.

The provision of the finely subdivided raw materials agglomerated intocompact rounded pellets, of 4 to 20 mm. maximum dimension, in accordancewith this invention, has enabled the economical production of highquality glass, while overcoming the deficiencies of prior practices andproposals.

The fact that each pellet contains a mixture of fine particles of theraw materials in intimate contact and has the desired composition,appears to be of primary importance in the attainment of rapidvitriiication and melting. The size range appears to be optimum withrespect to the retention of adequate heat conductivity so that thevitrication process is not signicantly retarded. It is a signicantcharacteristic of the pellets that they have high heat conductivity,wherefore the heat from the flame in the melting chamber requires theshortest possible time entirely to penetrate into the pellets within thegiven size range.

Furthermore, the crushing strength and handling characteristics of thepellets are highly satisfactory for commercial operations. Withoutadequate resistance to crushing, the pellets would disintegrate to iinepowders and create the problems and disadvantages described previously.Pellets in accordance with this invention have ade- -quate crushingstrength for easy handling as well as for ready transfer and storage ofthe pellets without dusting or segregation of ingredients.

It should be noted that the prior preparation of briquettes or sinteredlumps of the raw materials has not passed beyond the experimental stage,and has largely been discontinued not only because the anticipatedincrease in the speed of melting and vitriication has not been obtained,but also because of the high expense and other disadvantages entailed. Afurther disadvantage of prior art proposals for briquetting was therequirement for extraneous binders such as inorganic salts, for example,metal silicates or phosphates; or organic binders, such as phenolicresins.

Such extraneous binders are disadvantageous not only because of theadded expense but particularly because the introduction of undesiredimpurities and heterogeneity into the resulting glass created addedproblems.

In preparing the pellets in accordance with the present invention,adequate binding of the ingredient particles is due to two factors,namely, the fine particle sizes and the use of caustic soda in additionto water.

The attainment of the above-described advantages has been substantiatedin commercial operations in accordance with this invention. In oneinstallation, for example, the output of a glass furnace was therebyincreased from metric tons per day to 180 metric tons per day. Inanother case, the output was thereby increased from 110 metric tons tometric tons per day. The melting rate, defined as the ratio of furnacemelting area to the output in metric tons of glass per day, can readilyattain 2.4 square feet per ton per day. Likewise, the fuel consumptioncan be readily reduced to 150 liters of oil per ton of glass produced.

Thus, the present invention makes it possible to produce glass ofimproved quality at an increased capacity per unit melting area of theglass furnace. Furthermore,

the storage and transfer of pellets is readily effected without anysegregation of the ingredients and without the dispersion of dusts inthe furnace or in the working surroundings. Extended commercial use inaccordance with this invention has also confirmed the obtainment ofsatisfactory performance Without any unusual attack on or impairment ofthe furnace refractories.

What is claimed is:

1. In the preparation of a melting-furnace charge for producinghomogeneous glass, said charge including as main raw materialingredients silica, limestone, sodium carbonate and sodium hydroxide,the process of increasing the speed of vitrification of said chargecomprising the steps of:

(a) pulverizing said silica to particles finer than 0.25

v millimeter, at least 60% thereof being finer than than about 0.15millimeter;

(b) pulverizing said limestone and sodium carbonate to particles finerthan 0.42 millimeter, at least 65% thereof being finer than about 0.15millimeter;

(c) mixing said pulverized silica, limestone, and sdium carbonate;

(d) adding water and sodium hydroxide to said mixed pulverized silicalimestone, and sodium carbonate, said sodium hydroxide being added in aproportion furnishing at least 20% by weight of the total Na2O presentin the final analysis of the glass;

(e) further mixing the said pulverized silica, limestone, sodiumcarbonate, water and sodium hydroxide, said added water being in anamount whereby said mixed silica, limestone, sodium carbonate arefurther mixed in the presence of 12% to 20% by weight of Water to obtaina moist mixture;

(f) then successively discharging aggregates of said moist mixture totumble repeatedly over one another whereby to form rounded pelletsthereof the majority of said pellets having a size of about 4millimeters to 20 millimeters;

(g) then, drying said pellets at an initial heating temperature below200 C. and at a final heating temperature not exceeding 700 C.; and,

(h) recovering crush-resistant rounded tumble-formed pellets ofsubstantially uniform composition and having a size of about 4 to 20millimeters, said pellets being substantially free of any binder otherthan said glass-forming ingredients.

2. The process of claim 1, wherein said pellets are formed having awater content of about 15% 3. The process of claim 1, wherein saidpellets are dried at an initial temperature of 130 C. to 180 C.

4. In the preparation of a melting-furnace charge for producinghomogeneous glass, said charge including as main raw materialingredients silica, limestone and sodium hydroxide, the process ofincreasing the speed of vitrification of said charge comprising thesteps of:

(a) pulveriving said silica to particles finer than 0.25 millimeter, atleast thereof being finer than about 0.15 millimeter;

(b) pulverizing said limestone to particles finer than 0.42 millimeter,at least thereof being finer than about 0.15 millimeter;

(c) mixing said pulverized silica and limestone;

(d) adding water and sodium hydroxide to said mixed pulverized silicaand limestone, said sodium hydroxide being added in a proportionfurnishing at least 20% by weight of the total Na20 present in the finalanalysis of the glass;

(e) further mixing the said pulverized silica, limestone, water andsodium hydroxide, said added water being in an amount whereby said.mixed silica and limestone are further mixed in the presence of 12% to20% by weight of water to obtain a moist mixture;

(f) successively discharging aggregates of said moist mixture to tumblerepeatedly over one another whereby to form rounded pellets thereof, themajority of said pellets having a size of about 4 millimeters to 20millimeters;

(g) then drying said pellets at an initial heating temperature below 200C. and at a final heating temperature not exceeding 700 C.; and

(h) recovering crush-resistant rounded tumble-formed pellets ofsubstantially uniform composition and having a size of about 4 to 20millimeters, said pellets being substantially free of any binder otherthan said glass-forming ingredients.

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